PRIMER OF PRIMERS. 185 



sary for the growth of trees. Such a soil was 

 formed by the gradual disintegration of hard 

 rocks, and by the growth and subsequent decay 

 of thousands of generations of plants. 



Forests are generally found on the slopes of 

 mountains, where the rainfall is considerable and 

 well distributed throughout the year. The moun- 

 tains are, therefore, the natural home of the forest. 



Forests are found especially on that coast of an 

 island or continent which is exposed to the preva- 

 lent wind, because there the rainfall is considera- 

 ble and no extended time occurs when the rain is 

 absent. 



Soil was originally formed by the gradual disin- 

 tegration of the crystalline rocks that were pro- 

 duced by the cooling of the earth's crust. 



Disintegration of rocks is effected by various 

 causes, mainly, however, by the action, in some 

 way or another, of water. 



Sometimes the soil is found resting on the sur- 

 face of the rock from which it was derived by 

 disintegration. In such cases its general charac- 

 ter can be directly traced to the composition of the 

 underlying rocks by the gradual change which can 

 be observed from the loose, porous soil on top, to 

 the hard, untouched, virgin rock below. 



16* 



